In the operation of steam flooding to stimulate production of oil from oil reservoirs it is important to have a simple and accurate method to determine the quality of steam at the well head of an injection well. In such a stimulation process the amount of heat input to the reservoir determines the rate and amount of oil recovery, and heat input depends directly upon the steam quality. Steam that is generated for injection into the reservoir arrives at the well head as saturated or wet steam, i.e., a mixture of vapor and liquid, at super-atmospheric pressure. The greater the proportion of vapor in that mixture, i.e., the greater the steam quality, the more the heat input to the reservoir. Steam quality thus directly affects the rate and the ultimate amount of recovery of oil, and therefore has a bearing upon earnings and investment requirements.
Various methods have been used for determining steam quality in saturated steam lines with varying degrees of accuracy and simplicity of operation. One such method extracts a sample of steam from the steam line through a throttling orifice, separates the liquid from the vapor in a separator vessel, compares the salt concentrations in the liquid portion of the sample and in the feedwater to the steam generator, and solves equations relating the steam quality to those salt concentrations and to enthalpy values (from steam tables) for the liquid and vapor components of steam. In that method, which is described in U.S. Pat. Nos. 3,499,488, 3,596,516, and 3,550,849, assigned to the same assignee as this application, the physical separation of liquid and vapor occurs in a separator off to the side of the steam line with the resulting possibility of condensation within the separator under cold and windy ambient conditions. Such condensation invalidates the assumptions on which the method is based, thus compromising the accuracy of the measurement. Additionally in using the above described method it is difficult to get a consistently representative sample because of the complex nature of two-phase flow, and also problems arise from the deposition of dissolved solids in the throttling orifice. One such problem is that the sample so derived has an erroneously low concentration of dissolved solids; another problem is that the deposited solids tend to plug the orifice.